The spotlight powerplant is a 2.0-liter turbocharged four-cylinder making 270 hp and 260 lb-ft of torque. It uses a twin-scroll setup for a broad torque range.

This is a different, more efficient engine than the high-output 2.0-liter four-banger in the Buick Regal GS, which was used in the Pontiac Solstice and the Saturn Sky.

The ATS is the first General Motors vehicle to get the new 2.0-liter engine. It’s the second unit from the family of engines that debuted as the 2.5-liter engine on the Chevrolet Malibu.

The other ATS engines are the familiar 3.6-liter V6 used on the CTS and the SRX, and 2.5-liter naturally aspirated four-cylinder. The powertrains will be mated to six-speed automatic and manual transmissions.

The rear-wheel-drive ATS is Cadillac’s BMW 3-series fighter and is expected to be lighter than the 3-series. It will slot below the CTS in the Caddy portfolio and is expected to become the brand’s volume leader after it launches in the second half of 2012.

The car has been undergoing extensive testing on the Nürburgring this year, as engineers stretch its performance capabilities and tune the chassis.

“This has been a segment dominated by German cars, and as such we’ve developed a compelling competitor,” said Dave Leone, GM’s global vehicle line executive for rear-wheel and performance cars.